1. Field of the Invention
This invention relates to the chemical modification of paper and paperboard made of felted cellulose fibers to impart to the paper the property of water repellancy with a very low degree of substitution and without significantly altering the tensile strength of the treated product.
2. Description of the Prior Art
In the treatment or sizing of paper to improve the properties of the paper, it is conventional and well-known to add a sizing agent, such as, rosin, wax, asphaltic material and the like to the pulp slurry before the sheet is formed. Other agents, such as organic ketene dimers are also used, mostly in the internal sizing process. This art is found in a number of U.S. and foreign patents, namely, U.S. Pat. Nos. 2,865,743; 3,130,118; 2,961,366; 2,986,488; 2,961,367; 3,046,186; 3,524,796; 3,070,452; 2,762,270; 2,785,067; 2,627,477; 3,212,961; 3,223,544; 3,006,806; Canadian Patents 611,247; 688,272; 567,352; 816,411; 846,762; and British Patents 802,356; 802,357; and 985,028. In addition, a good discussion of the art is found in Fiber Science Series, Volume 4, "Chemical Modification of Papermaking Fibers" by Kyle Ward, Jr., Institute of Paper Chemistry, Published by Marcel Dekker, Inc., N.Y., 1973.